Since a water-soluble oxygen-containing compound having a phenolic hydroxyl group or a carbonyl group in a process wastewater discharged from a process for the production of a phenol or the like, is contained, the wastewater cannot be discharged without treating. Therefore, it is generally discharged after distillation separation and removal of the oxygen-containing compound having a phenolic hydroxyl group or a carbonyl group by a treatment such as a biological treatment after diluting it with a large amount of water or a treatment of burning incineration. By the way, when phenol as a worthy component is separated and purified from a reaction mixture, regarding a method for separating an oxygen-containing compound having a carbonyl group contained as by-products in the reaction mixture by distillation, various improvements are conducted (For example, see JP-A-2004-515484). But, as trials for reducing a load of post-treatment step of a wastewater through separation of a trace amount of by-products contained in a process wastewater by distillation, a purification-separation method of water-phenol by two-columns system using azeotropic distillation, is only proposed (For example, see JP-A-09-253624), and, reduction of an energy cost for distillation of a wastewater with respect to removal of an oxygen-containing compound having a phenolic hydroxy group or a carbonyl group as a by-product, and as a method for reduction of a load of an oxygen-containing compound having a phenolic hydroxyl group or a carbonyl group in a biological treatment are not described in the above document, and increased rationalization has been required.